Structural origins of enhanced capacity retention in novel copolymerized sulfur-based composite cathodes for high-energy density Li-S batteries
Vladimir P. Oleshko, Jenny J. Kim, Jennifer L. Schaefer, Steven D. Hudson, Christopher L. Soles, Adam Simmonds, Jared Griebel, Kookheon Char, Jeff Pyun
Poly(sulfur-random-(1,3-diisopropenylbenzene) (poly(S-r-DIB)) copolymers synthesized via inverse vulcanization form high molecular mass electrochemically active polymers capable of realizing enhanced capacity retention (1005 mAh/g at 100 cycles) and lifetimes over 500 cycles as cathodes for Li-S batteries. As a cross-linking agent, DIB is responsible for generation of lithiated organosulfur products Li4(Sx)4-DIB (x~8), which appear to prevent the irreversible deposition of insoluble lower Li polysulfides generated during cycling, thus suppressing detrimental shuttle mechanism and improving the long term battery performance. To clarify origins of the enhanced capacity retention, we characterized morphological and compositional features created when combining the poly(S-r-DIB) copolymers, with varying DIB content (0-50 % by mass), with conductive carbon to form functional Li-S cathodes. This characterization is done both in the pristine state as well as after the transformations that occur during charge-discharge cycling. The use of elemental sulfur in the composite cathode leads to heterogeneous aggregates of carbon nanoparticles and poor mixing with the sulfur, forming a loosely percolated network of electrically conductive pathways and extended micro- and mesoscale porosity. The incorporation of the poly(S-r-DIB) copolymers tend to embed carbon nanoparticles more intimately in the sulfur containing copolymer because of a stronger cohesion between the components as compared to the conventional sulfur-carbon cathode. This increases the compositional homogeneity in the cathode and apparently increases the interfacial area between the electrochemically active components and improves physico-mechanical stability of the composite cathode and leads to both increased capacity and enhanced cycle life in a full battery assembly.